Switchgrass (Panicum virgatum L.) is a perennial native grass currently being investigated for use in production of cellulosic ethanol. Biofuel (cellulosic ethanol) from switchgrass has a low production cost and results in 540% more renewable energy than nonrenewable energy consumed (Schmer et al. 2008). Additionally, estimated greenhouse gas emissions from switchgrass-based cellulosic ethanol are 94% lower than estimated greenhouse gas emissions from gasoline (Schmer et al. 2008). The ethanol is produced through a biochemical process that uses various enzymes to convert the switchgrass cellulose to ethanol. To maximize the efficiency of this process, the plant extractives can be removed to allow for optimum enzyme activity (N. Labbé, personal communication, Anderson and Akin 2007). Switchgrass extractives are high in phenolic compounds (such as p-coumaric, ferulic, and sinapic acids), which are associated with the induction of the systemic resistance response that plants exhibit in the presence of pathogens or other stresses (Chen et al. 2010).
Although switchgrass-derived cellulosic ethanol production is a main focus of the biofuels initiative in Tennessee, limited research has been done on plant pathogens that occur on switchgrass grown in the state. As expansive monocultures of switchgrass are developed for commercial production, disease pressure is likely to increase due to lack of plant biodiversity (Wolfe 2000, Etheridge et al. 2001).
Host: Panicum virgatum 
Switchgrass (Panicum virgatum L.) is a perennial, warm-season (C4) tall prairie grass that has been adopted as a crop in the last 50 years (Parrish and Fike 2005). Switchgrass can grow up to 3 meters in height in one growing season and possesses a diffuse panicle seedhead with two-flowered spikelets at the end of long branches (Bouton 2008). Most switchgrass genotypes are caespitose (grow in small, dense clumps) and develop short rhizomes that allow the plant to form a loose sod over time (Bouton 2008). Switchgrass is native to North America and is adapted to a wide geographical range, spanning from 20° to 60° north latitude and east of 100° west longitude to the Atlantic seaboard (Vogel 2004). Two predominant ecotypes have developed: lowlands and uplands (Brunken and Estes 1975, Porter 1966, Vogel 2004). Basic chromosome number for both ecotypes is 9, but the somatic chromosome number is usually tetraploid in lowland selections and octaploid in uplands (Bouton 2008). Soil acidity typically is not a factor in switchgrass growth, but increased water-holding capacity as related to soil texture is necessary for stand establishment (Parrish and Fike 2005).
Several studies have demonstrated the importance of vesicular-arbuscular mycorrhizae in nutrient uptake for switchgrass (Clark and Zeto 2000, Hetrick et al. 1988, Boerner 1992, Brejda et al. 1998, Wilson et al. 2001). Brejda et al. (1998) tested growth and nutrient uptake in response to natural versus sterilized rhizosphere soil conditions. Switchgrass grown in soil with rhizosphere fungi and bacteria produced 15-fold greater overall biomass, showed 6-fold greater nitrogen recovery and 36-fold greater phosphorus recovery than plants grown in sterile soil (Brejda et al. 1998). In several reviews, it has been suggested that mycorrhizae may mediate plant responses to drought stress, nutrient deficiencies, toxic metals, and pathogen attack (Parrish and Fike 2005).
Pathogens
More than 75 fungal pathogens occur on switchgrass in the United States (Farr and Rossman 2011). Close to 150 fungal isolates have been identified on switchgrass, but pathogenicity of many of these has not been determined (Ghmire et al. 2011, Gravert and Munkvold 2002). Twenty-four species of plant-parasitic nematodes, including species of Dorylaimida, Triplonchida, and Tylenchida and five viruses (Panicum mosaic virus, Barley yellow dwarf virus, Sugarcane mosaic virus, Wheat soil-borne mosaic virus, and Maize rayado fino virus) have been reported on switchgrass in the United States (Agindotan et al. 2010, Cassida et al. 2005, Farr and Rossman 2011, Garrett et al. 2004, Mekete et al. 2011, Sill 1957). In Tennessee, prior to published results of the current study, only one fungal disease had been reported: rust caused by Puccinia emaculata (Zale et al. 2008). The lack of reports of pathogens in Tennessee does not indicate a lack of presence. In 2008, researchers at the University of Tennessee found Tilletia pulcherrima (the causal agent of bunt disease) on switchgrass seed that had been produced in Texas and distributed to growers in Tennessee (Carris et al. 2008). Based on reports from other regions in the United States (Farr and Rossman 2011), genera of fungal pathogens that are likely to occur in Tennessee include Alternaria, Bipolaris, Curvularia, and Fusarium. 
Alternaria species can be difficult to identify due to conidial plasticity and low genetic variability among species (Misaghi et al. 1978, Kusaba and Tsunge 1995, Pryor and Gilbertson 2000). Alternaria includes nearly 100 species of dematiaceous mitosporic fungi that occur worldwide and range from general saprophytes to specific plant pathogens of cereals, ornamentals, nuts, vegetables, and fruits, including citrus (Thomma 2003). Spore production can be induced in the laboratory by use of specific culture media and environmental conditions (Shahin and Shephard 1979). Differential media assays have been developed for some Alternaria spp. (Andersen et al. 2001). Molecular identity can be confirmed by analysis of the internal transcribed spacer (ITS) region and mitochondrial small subunit (SSU) ribosomal DNA (Kusaba and Tsunge 1995, Pryor and Gilbertson 2000), or by random amplified polymorphic DNA (RAPD) analysis (Pryor and Gilbertson 2002, Roberts et al. 2000). Some anamorphic Alternaria species are associated with the teleomorphic genus Lewia, which is a member of the family Pleosporaceae in the phylum Ascomycota (Kirk 2011).
The genus Bipolaris (teleomorph: Cochliobolus) includes over 45 species that range from economically important pathogens of monocotyledonous hosts, such as wheat, barley, rice, and corn, to opportunistic human pathogens (Choudhry et al. 2010). On switchgrass, B. oryzae, B. sorokiniana, and B. spicifera have been described as pathogenic in the United States (Farr and Rossman 2011, Krupinsky et al. 2004). Various species of Bipolaris are known to be seedborne and in 2004, B. spicifera was reported on grass seed exported from the United States to Korea (Koo et al. 2004).
Species of the genus Fusarium (teleomorph: Gibberella spp., Nectria spp., Calonectria spp., and Micronectria spp.) are a diverse array of mitosporic fungi, many of which are phytopathogenic to a wide range of plants under different environmental conditions (Booth 1971, Doohan et al. 2003). Fusarium species cause economically important diseases of monocotyledonous hosts such as Fusarium head blight of wheat and ear rot of corn (Parry et al. 1995, Sutton 1982). Some nonpathogenic Fusarium species are endophytic and can be added to soil to protect plants against fungal pathogens, including pathogenic species of Fusarium (Kavroulakis et al. 2007). Fusarium species have also been shown to have activity in Fusarium wilt-suppressive soils (Weller et al. 2002).
The genus Curvularia (teleomorph: Cochliobolus) includes species pathogenic to a wide range of hosts including wheat, rice, yam, mango, citrus, coconut and sorghum (de Luna et al. 2002). Curvularia spp. can be seedborne or soilborne and can cause primary infections, secondary infections, and post-harvest diseases (Lockwood 1988, Meehan 1947, Ray and Raavi 2005). Curvularia lunata has been found on grass seed exported from the United States to Korea (Koo et al. 2004) and C. geniculata has been identified causing secondary leaf spot on switchgrass in Kansas and Nebraska (Anonymous 1960).
In addition to their potential for decreasing crop yields, pathogens of switchgrass also are being investigated for use in the degradation of plant material in the conversion process of plant biomass to ethanol (Gibson et al. 2011). Plant pathogens may be pre-adapted for this new use as they have evolved numerous ways to degrade plant cell walls. Research is currently being conducted to exploit these mechanisms and enzymes for use in lignocellulose digestion of second-generation biofuels crops such as switchgrass.
During the conversion process from cellulose to ethanol, the switchgrass biomass undergoes a series of biochemical reactions during which the switchgrass extractives are inhibitory to the process (Thammasouk et al. 1997). The ethanol-soluble switchgrass extractives contain phenolic compounds such as p-coumaric, ferulic, and sinapic acids, which are associated with antimicrobial activity, antioxidant activity, UV protection, and the induction of the systemic resistance response that plants exhibit in the presence of pathogens (Chen et al. 2010, Graf 1992, Herald and Davidson 1983, Kikuzaki et al. 2002, Nicholson and Hammerschmidt 1992, Walker 1994). In humans and mice, these phenolic compounds have also been shown to have anti-inflammatory activity, cholesterol-lowering capacity, and the ability to boost natural immune defenses (Chawla et al. 1987, Hu et al. 1990, Liu 1987).
Phenolic compounds can prevent plant diseases by inducing plant defense responses or by preventing pathogen growth (Nicholson and Hammerschmidt 1992). The responses of plants to pathogens have been differentiated based on host and non-host interactions, both of which are characterized by the early accumulation of phenolic compounds at the infection site (Fernandez and Heath 1989, Heath 1980). Commercial products are available that use plant extractives to induce resistance against both bacterial and fungal plant pathogens (Marrone BioInnovations 2009, Randoux et al. 2006). These products can be used in both conventional and organic crop production. For instance, Randoux et al. (2006) used giant-knotweed extract to inhibit fungal growth and enhance plant defenses in wheat inoculated with Blumeria graminis f. sp. tritici (Randoux et al 2006).
Switchgrass (Panicum virgatum L.) is a perennial grass currently being investigated for use as a biomass feedstock for cellulosic ethanol production. Switchgrass biofuel has a low production cost and yields much more renewable energy than nonrenewable energy consumed in the process of production. Also, estimated greenhouse gas emissions from switchgrass-derived ethanol are 94% lower than estimated greenhouse gas emissions from gasoline (Schmer et al. 2008).
Although switchgrass-derived cellulosic ethanol production is a main focus of the biofuels initiative in Tennessee, only limited research has been done on pathogens that occur on switchgrass grown in the state. As expansive monocultures of switchgrass are developed for commercial production, disease pressure is likely to increase due to lack of plant biodiversity in those fields.
Previous studies have shown that nearly 150 species of fungi occur on switchgrass; with 75 confirmed pathogens in the United States (Fan and Rossman 2011). Prior to this study, only one fungal disease had been reported in Tennessee: rust caused by Puccinia emaculata (Zale et al. 2008). The lack of reports of switchgrass pathogens in Tennessee does not indicate a lack of presence. In 2008, researchers at the University of Tennessee found Tilletia pulcherrima (causal agent of bunt disease) on switchgrass seed that had been produced in Texas and distributed to growers in Tennessee (Carris et al. 2008). The purpose of this research was to develop information on fungal pathogens that decrease crop yield and quality of switchgrass grown in Tennessee.
Switchgrass (Panicum virgatum L.) is a warm-season grass that, due to its ability to adapt to a wide variety of environmental conditions, low fertility requirements, and low production cost, is being grown for biofuel production across the US and around the world. Although switchgrass is a perennial plant, stand establishment has proven to be a problem for growers. One factor contributing to the problem of stand establishment is poor seed quality (Parrish and Fike 2005; Sanderson et al. 2006). To our knowledge, only two studies have been conducted to examine switchgrass seed for pathogens (Carris et al. 2008, Tomaso-Peterson and Balbalian 2010).
In our studies, we have observed increased stand establishment and enhanced growth and vigor with surface-sterilized seed versus untreated seed (unpublished data). Currently, no seed certification program exists for switchgrass. The objective of this study was to identify major seedborne pathogens, their incidence in diverse seed lots, and the current geographic distribution of these organisms as determined by seed source location.